There is a widespread need for inspection data for electronic parts in a manufacturing environment. One common inspection method uses a video camera to acquire two-dimensional images of a device-under-test.
Height distribution of a surface can be obtained by projecting a light stripe pattern onto the surface and then reimaging the light pattern that appears on the surface. One technique for extracting this information based on taking multiple images (3 or more) of the light pattern that appears on the surface while shifting the position (phase) of the projected light stripe pattern is referred to as phase shifting interferometry, as disclosed in U.S. Pat. Nos. 4,641,972 and 4,212,073.
The multiple images are usually taken using a CCD (charge-coupled device) video camera with the images being digitized and transferred to a computer where phase-shift analysis, based on images being used as “buckets,” converts the information to a contour map (i.e., a three-dimensional representation) of the surface.
The techniques used to obtain the multiple images are based on methods that keep the camera and viewed surface stationary with respect to each other while moving the projected pattern.
One technique for capturing just one bucket image using a line scan camera is described in U.S. Pat. No. 4,965,665.
U.S. Pat. Nos. 5,398,113 and 5,355,221 disclose white-light interferometry systems which profile surfaces of objects.
In U.S. Pat. No. 5,636,025, an optical measuring system is disclosed which includes a light source, gratings, lenses, and camera. A mechanical translation device moves one of the gratings in a plane parallel to a reference surface to effect a phase shift of a projected image of the grating on the contoured surface to be measured. A second mechanical translation device moves one of the lenses to effect a change in the contour interval. A first phase of the points on the contoured surface is taken, via a four-bucket algorithm, at a first contour interval. A second phase of the points is taken at a second contour interval. A control system, including a computer, determines a coarse measurement using the difference between the first and second phases. The control system further determines a fine measurement using either the first or second phase. The displacement or distance, relative to the reference plane, of each point is determined, via the control system, using the fine and coarse measurements.
Current vision inspection systems have many problems. Among the problems are that the intensity of the light received at a light receiver may vary. When the intensity of the light varies at the receiver, it is very difficult to correlate signals that are generated by the receiver. Another problem is that the light receiver may vary in temperature. Many of the receivers use a charge coupled device (also called a CCD). A charged coupled device or CCD is a high speed, high density computer storage medium in which the transfer of stored charges provides the information. The stored charges produce signals which provide image information received. CCDs tend to heat up as they operate. As the CCDs heat, the signals produced shift or vary. Typically, there are currents associated with dark areas and currents associated with light areas. Part of correlating the data obtained includes a subtraction of the current associated with a dark area. This is used to find the absolute value of the current associated with a light area. Dark currents approximately double with every 7° C. rise in temperature. When the currents associated with dark areas get too large, the current shows up as noise. The elimination of noise is another problem which complicates processing of the image. Within a CCD there are many individual sources of light. These light sources, typically LEDs, may also vary on a pixel-by-pixel or source-by-source basis since the gain and bias may differ as a result of manufacturing variances. This too adds to the difficulty in correlating data obtained from one sensor or another sensor since light of different intensities is being produced by the source.
Another problem is that infrared light may also adversely effect the sensor. Infrared light can vary the signal strength read by the sensor. Yet another problem associated with machine vision systems is that an image is captured using a substantially constant source of light. In order to get an adequate image, the image must be captured over a relatively long time frame. The result is the same as occurs when there is low levels of light and the object to photograph is moving fast. The image is blurred or smeared.
To overcome the problems stated above as well as other problems, there is a need for an improved machine-vision system and more specifically there is a need for a circuit or circuits which can be used to control lamp brightness or the aperture associated with a CCD. There is also a need for temperature control of the sensor to prevent or lessen the effects of thermal drift on the bias or gain of the signal output from the sensor. In addition, there is a need for a system which allows for correction of a CCD or other light sources on a pixel-by-pixel basis so that manufacturing tolerances are accounted for. There is also need for a device that controls the amount of infrared light that is received at a sensor so as control the bias or gain of the signal output from the sensor. In addition, there is a need for a device which facilitates automated high-speed three-dimensional inspection of objects.